1. Field of the Invention
This invention relates generally to a method for starting an engine of a hybrid electric vehicle (HEV) by transmitting torque from a starting motor through a clutch and damper to the engine.
2. Description of the Prior Art
A modular hybrid transmission (MHT) is an arrangement of powertrain components that includes an internal combustion engine, torsion damper, disconnect clutch, electric motor/generator and torque converter arranged in series ahead of an automatic transmission. The electric machine operates as a motor to crank the engine during starting using a high voltage battery as a source of power for the motor.
In an MHT system a key objective is to start the engine consistently and quickly using the least amount of starting motor reserve torque. Using a one-size fits all disconnect clutch pressure profile may produce engine starts with varying accelerations, which presents problems such as fuel/air cranking calibrations, possible no-starts, etc. Increased starting torque requires more reserve starting torque from the starting motor.
Varying the operative pressure profile of the disconnect clutch based on speed is likely difficult to control due its reliance on information that becomes available too late for ideal application to the engine stating procedure. Furthermore, a vehicle system controller (VSC) controls electric machine torque and possibly speed. A disconnect clutch pressure controller adjusts clutch actuation pressure based on engine speed or acceleration, thereby producing the potential for engine speed control difficulty. A VSC receives input from the vehicle operator, coordinates the engine and electric machine, and may disconnect the clutch and transmission.